24V, 15A Step-Down Regulator in Sub-1mm Height Package Pushes Monolithic Performance Limits Stephanie Dai Monolithic switching regulators and switching controllers together dominate the DC/DC converter market. Generally, there is little overlap in their respective applications. Controller-based solutions are favored for high performance, higher power applications where minimal power loss and top thermal performance are priorities. In contrast, monolithic regulators are favored in lower power applications where compact size is the main requirement. Controllers typically offer more features than monolithic solutions, but are at a significant solution-size disadvantage. The light footprint of monolithic regulators usually comes at a cost of features and increased power loss, and their reliance on integrated MOSFETs places practical limits on power. The LTC3613 monolithic regulator blurs the line drawn between applications for controllers and monolithic regulators by combining a high performance fully featured controller with onboard low RDS(ON) MOSFETs. FEATURES The LTC3613 can accept an input voltage between 4.5V to 24V and supports output voltages between 0.6V to 5.5V. The onboard top and bottom MOSFETs feature low RDS(ON), around 7mΩ and 5mΩ, respectively, keeping power dissipation low and allowing the LTC3613 to deliver up to 15A of adjustable load current. The LTC3613 features true remote differential output voltage sensing. This allows for accurate regulation of the output with maximum load currents and shared ground planes. This feature is critical for low output voltage applications, where even small voltage offsets caused by parasitic IR drops on board traces can cost several percentage points in regulation accuracy. Remote differential output sensing and the LTC3613’s accurate internal reference combine to offer excellent output regulation accuracy over line, load and 26 | July 2012 : LT Journal of Analog Innovation temperature: ±0.25% at 25°C, ±0.67% from 0°C to 85°C and 1% from –40°C to 125°C. frequency is constant over steady state conditions under line and load. It also allows the LTC3613 to recover from a large load step in only a few short cycles. This architecture yields well balanced current sharing among multiple LTC3613s, which can be easily paralleled for high power applications. It also includes a phaselock loop (PLL) for synchronization to an The LTC3613 has a low minimum on-time of 60ns, allowing for high step-down ratios at high switching frequencies. Because of its sophisticated controlled ontime, valley current mode architecture, the on-time is controlled so that the switching Figure 1. 24V input to 1.2V output using inductor DCR sensing to minimize solution size and cost and to maximize efficiency LTspice IV INTVCC RPGD 100k RDIV1 52.3k PVIN 350kHz CSS 0.1µF CITH1 220pF RITH 28k CIN2 10µF VOUT PGOOD LTC3613 VRNG RDIV2 10k circuits.linear.com/560 SVIN RUN SENSE– SENSE+ MODE/PLLIN EXTVCC SW CDCR RDCR 0.1µF 3.09k L1 0.56µH VOUT 1.2V 15A CB 0.1µF BOOST TRACK/SS DB INTVCC ITH INTVCC CVCC 4.7µF CITH2 100pF RT 115k VIN CIN1 6V TO 24V 82µF 25V + RFB2 20k RFB1 20k COUT2 100µF ×2 + PGND RT SGND VOSNS+ VOSNS– CIN1: SANYO 25SVPD82M COUT1: SANYO 2R5TPE330M9 3613 F10 DB: CENTRAL CMDSH-3 L1: VISHAY IHLP4040DZ-056µH COUT1 330µF 2.5V ×2 design ideas LTC3608 LTC3609 LTC3610 LTC3611 LTC3613 PV IN(MAX) 18V 32V 24V 32V 24V I LOAD(MAX) 8A 6A 12A 10A 15A Frequency Sync Precise Differential Output Sensing ±1% ±1% ±1% ±1% ±0.67% Accurate Current Sensing Bottom FET R DS(ON) Bottom FET R DS(ON) Bottom FET R DS(ON) Bottom FET R DS(ON) R SENSE or DCR sensing MOSFET R DS(ON) Top/Bottom 10mΩ/8mΩ 18mΩ/13mΩ 12mΩ/6.5mΩ 15mΩ/9mΩ 7.5mΩ/5.5mΩ Package 7mm × 8mm × 0.9mm 64-pin 7mm × 8mm × 0.9mm 64-pin 9mm × 9mm × 0.9mm 52-pin 9mm × 9mm × 0.9mm 52-pin 7mm × 9mm × 0.9mm 56-pin Table 1. High power monolithic regulator family external clock, especially beneficial for high current, low output voltage applications where interleaving of paralleled phases can minimize output voltage ripple. 50%, then the maximum sense voltage is reduced to about one-fourth of its full value, limiting the inductor current level to one-fourth of its maximum value. The LTC3613 includes several safety and protection features including overvoltage protection and current-limit foldback. If the output exceeds 7.5% of the programmed value, then it is considered an overvoltage (OV) condition, the top MOSFET is immediately turned off and the bottom MOSFET is turned on indefinitely until the OV condition is cleared. A power good output monitor is also available which flags if the part is outside a ±7.5% window of the 0.6V reference voltage. In the case of an output short circuit, if the output fails by more than The LTC3613 offers precise control of the output during start-up and shutdown sequencing though its output voltage tracking and soft-start features. An external VCC input pin is also available, allowing for bypassing of its internal LDO for an efficiency benefit in high power applications. Figure 2. Efficiency of the regulator in Figure 1 Figure 3. Load transient response of the circuit in Figure 1 The LTC3613 can be configured to sense the inductor current through a series sense resistor, RSENSE , or via an inductor DCR sensing network. The tradeoffs between the two current sensing schemes are largely matters of cost, power 100 90 EFFICIENCY (%) 80 PULSE-SKIPPING MODE VOUT 100mV/DIV 70 60 50 FORCED CONTINUOUS MODE IL 10A/DIV 40 30 20 VIN = 12V VOUT = 1.2V 10 0 0.1 1 10 LOAD CURRENT (A) 100 ILOAD 10A/DIV 40µs/DIV LOAD TRANSIENT = 0A TO 15A VIN = 12V, VOUT = 1.5V FIGURE 10 CIRCUIT consumption and accuracy. DCR sensing owes its increasing popularity to its lower expense and power loss compared to a sense resistor scheme. Even so, the tight tolerances of current-sensing resistors provide the most accurate current limit. Figure 1 shows a typical application of the LTC3613, configured for DCR sensing in a high step-down solution, 24V to 1.2V, and synchronized to a 350kHz external clock. Figure 2 shows the efficiency and Figure 3 shows transient performance. CONCLUSION The LTC3613 offers far more design flexibility than a typical monolithic switching regulator by including a variety of userprogrammable features such as soft-start, programmable frequency, external clock synchronization, adjustable current limit and selectable light load operating modes. Its critical safety features such as overvoltage protection and programmable current limit with foldback current limiting further improve the robustness of the part. An external VCC input is provided for high power applications. Its compact solution size, extensive feature set and high performance capabilities extend its range of use compared to traditional monolithics, making it suitable for an an expanding range of applications. n July 2012 : LT Journal of Analog Innovation | 27